1.1 Field Of The Invention
The present invention pertains generally to atomic absorption and plasma spectrometers, and more particularly, to spectroanalytical systems of the type that sense a special characteristic of a flame by absorption or emission technology. The present invention more specifically pertains to an assembly for reducing the amount of leakage in a plasma torch.
1.2 Description of the Prior Art
In atomic absorption and plasma spectroscopy, measurement of the absorption or emission of radiation at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample. Presently, the most common technique for atomizing an element for purposes of absorption measurement is by introducing a liquid sample containing the element of interest into a gas burner wherein droplets of the sample are vaporized and the elements ultimately atomized so as to form a radiation beam. In plasma emission systems, for example in an inductively coupled plasma (ICP) spectrometer, a liquid sample is nebulized with a plasma gas such as argon, nitrogen, etc. The sample liquid is suspended in microsized droplets which are introduced into the plasma torch wherein the atoms of the liquid are energized from the plasma. Energy emitted by the energized sample is measured by the spectroanalytical system.
In atomic absorption or emission type spectroanalytical systems, the material to be analyzed is introduced into a premix or gas flow chamber by a nebulizing arrangement using a regulated plasma gas or oxidant stream. The plasma gas or oxidant stream is ideally introduced into the gas flow chamber as a fine uniform spray of minute droplets, which droplets are entrained with a combustible fuel or plasma gas and flow through the gas flow chamber into a burner or plasma torch. Upon combustion, the combustible fuel energizes the material to be analyzed for purposes of analysis. In plasma units, the plasma torch energizes the material. An example of a nebulizer arrangement is seen in U.S. Pat. No. 4,220,413.
In ICP systems, the liquid sample is introduced into a plasma torch having a quartz chimney through a specially designed injector tube. Generally, a single assembly serves as both a support (or holder) for the quartz chimney and a mount for the injection tube. This assembly is generally referred to herein as the torch support assembly.
Because of the extreme heat associated with ICP systems, it is important to allow for the heat expansion of the quartz chimney, the injector tube, and their supporting apparatus. Absent such allowance, the quartz chimney and its supporting apparatus are prone to break when subjected to extreme temperatures. This is undesirable both because of the expense in labor and parts of repairing or replacing the broken parts and the opportunity costs lost when the ICP system is down. Additionally, because quartz chimneys are relatively fragile, frequent repair or replacement of torch support assemblies usually results in breakage of several chimneys, which are often to expensive to replace. Thus, one desirable attribute for torch support assemblies is to provide for the heat expansion of the quartz chimney, the injector tube and their supporting apparatus.
Another desirable attribute of torch support assemblies is to inhibit the formation of leaks. Generally, when a leak occurs (i.e., when the plasma gas is exposed to air) arcing is likely to occur. Such arcing usually results in damage to the support assembly to such a degree that the support assembly must be replaced or repaired. As discussed above, this replacement or repair time is disadvantageous in terms of real costs, broken chimneys, and lost opportunity costs.
Given the importance of manufacturing and set-up costs it is also desirable that torch support assemblies be easily manufactured and assembled.
In an attempt to implement the desirable attributes discussed above, a number of prior art torch support assemblies have been developed. These assemblies may be divided into three basic groups: (1) those having non-segmented, blown quartz torch and chimney assemblies ("Quartz assemblies"); (2) those having segmented attachments for the quartz chimney, the plasma gas inlet, the auxiliary gas inlet, and the injection tube ("Type I assemblies"); and (3) those having two segmented attachments: one for the quartz chimney, the plasma gas inlet, and the auxiliary gas inlet; and another for the injection tube ("Type II assemblies").
1.2.1 Prior Art Quartz Assemblies
The Quartz assemblies generally comprise a unitary, non-segmented blown quartz element having a quartz injection tube, inlets for plasma and auxiliary gas, and a quartz chimney. One such torch support assembly is the Quartz Torch for ICP, PE No. 0047-2048, manufactured by Perkin-Elmer. An example of a Quartz assembly is illustrated in FIG. 1. In this type of assembly, the connection between the injection tube of the quartz element and the supply of the liquid spray sample is made through a ball and socket connector where one end of the unitary torch assembly, i.e., the ball, is brought into direct contact with a ball joint adaptor that is connected to the supply of the spray sample.
Since the quartz assemblies consist of a single, blown quartz element, they cannot accommodate alumina injection tubes. This is disadvantageous in that the melting or fusion point of an quartz injector tube is much less than that for an alumina one. As a result, when the injection tube of a quartz assembly becomes plugged or clogged it likely to melt or fuse together--destroying the integrity of the torch and requiring complete replacement of the quartz assembly.
Another disadavantage of quartz assemblies is that they are prone to leak if not precisely machined with very small tolerances. The unitary design of the quartz assemblies restricts the likely regions of gas leakage to the plasma and auxiliary gas ports and the ball joint and ball joint adaptor. Because the integrity of the seal at the ball joint depends an a close fit between the ball of the quartz assembly and the socket of the ball joint adaptor, it is essential that the quartz ball and the ball joint adaptor be machined to very precise tolerances. The requirement for such precise machining generally increases both the cost of Quartz assemblies and the likelihood that a particular assembly will leak.
1.2.2 Prior Art Type I Assemblies
In an attempt to overcome the shortcomings of quartz assemblies and to accommodate alumina injector tubes, Type I torch assemblies were developed. These assemblies generally comprise a quartz chimney, a upper tube guide including a plasma gas port and a chimney support, and a lower tube guide including an auxiliary gas port. Such assemblies accommodate alumina injector tubes which are supported by both the upper and lower tube supports. One Type I assembly is the Plasma II and Plasma 40 Type I Demountable Torch Assembly, PE No. 58-0161, manufactured by Perkin-Elmer. An example of a Type I assembly is provided in FIG. 2.
As illustrated in FIG. 2, Type I assemblies rely on three compressed O-rings to inhibit gas leakage--a first compressed O-ring to provide a seal between the outer portion of the quartz chimney and the upper tube guide; a second compressed O-ring to provide a seal between the upper and lower tube guides; and a third compressed O-ring to provide a seal between the injector tube and the tube port in the lower tube guide. The O-rings are compressed by placing a shield over the chimney/tube guide combination, affixing the shield to a base having a bore therethrough, and forcing a compression plug into the base. Because the seals in Type I assemblies rely on the compression of the three O-rings, it is often necessary to use a special tool to force the compression plug into the base.
There are several disadvantages associated with Type I assemblies, the most obvious being the extensive amount of parts. This large number of parts results in extended assembly time. Additionally, the many interfaces between these parts provide many regions where gas leakage can occur. As discussed above when the plasma or auxiliary gas leaks, i.e., makes contact with atmospheric air, the plasma is likely to arc to the site of the leak. Such an arc may carbonize that site resulting in either extinction of the plasma torch or analytical results that have a high coefficient of variation.
Another disadvantage of Type I assemblies are their reliance on O-ring compression for sealing. As noted above, a compression plug is forced into the base to urge the shield towards the upper tube guide, compressing the first O-ring; to urge the upper tube guide towards the lower tube guide, compressing the second O-ring; and to urge the injection tube towards the compression plug, compressing the third O-ring. Often the amount of pressure that must be applied by the compression plug is quite high. In fact, most prior art Type I assemblies require a special tool for affixing the compression plug to the base.
While tension on the compression plug results in compression of the O-rings, it also results in tension on the quartz chimney and the injector tube. Too much pressure on the chimney or the injector tube results in breakage. Because there is little indication (other than experience) that the compression plug is being improperly tightened, attempts to properly assemble a Type I assembly, or to fix a leak by tightening the compression plug, frequently result in breakage of either the quartz chimney, the injector tube, or both. Such breakage necessitates a disassembly of the torch and a replacement of the broken part.
Still another disadvantage of the Type I assembly is that it is inherently prone to O-ring failure. One inherent attribute of ICP torches is that they become extremely hot and give off extensive UV radiation during operation. As illustrated in FIG. 2, the first O-ring is situated in close proximity to the plasma gas flow and the region where the sample exits the injector tube. This location exposes the first O-ring to extensive heat and UV radiation given off by the torch at the point where the plasma gas and the sample spray combine. As a result of this exposure the sealing qualities of the O-ring are prone to degrade over time and cause leaks. Such leaks either require disassembly of the torch and replacement of the failed O-ring (with the repair costs and associated downtime) or a further tightening of the compression plug (with the risk of breaking the chimney or injector tube).
A still further disadvantage of the Type I assemblies is that they often provide inaccurate or erratic test results. As mentioned above, the Type I assemblies can accommodate alumina injector tubes. Thus, unlike Quartz assemblies, meltdown of the injector tube for Type I assemblies is uncommon. Type I assemblies, however, are often plagued by leakage due to seal failure at the bottom of the injector tube port. As illustrated in FIG. 2 a single O-ring is used to create a seal between the injector tube and the base. If this seal fails, inaccurate test results (due to non-uniformity in the sample flow) or plugging of the injector tube (eventually resulting in the extinction of the plasma) may occur. Since only a single O-ring is used to seal the injector tube, failure of that O-ring may destroy the integrity of the entire torch. Additionally, because the sealing potential of the O-ring rests on the amount of pressure applied by the compression plug, improper assembly of the torch (e.g, too little pressure on the compression plug) may result in improper operation.
A final disadvantage of the Type I assemblies is that is essentially impossible to adjust the height of the injector tube in relation to the quartz chimney. As illustrated in FIG. 2, the seal of the injector tube is effected by forcing the third O-ring against the base, while at the same time forcing the injector tube against a hard non-resilient lip of the bore in the base. Because support and sealing for the injector tube is obtained ony when the top of the large diameter portion of the injector tube is in contact with the lip, it is essentially inpossible to adjust the height of the injector tube. This inability to adjust the tube often results in less that optimal test results.
1.2.3 Prior Art Type II Assemblies
In addition to the Quartz and Type I assemblies, a third type of assembly, the Type II assembly exists in the prior art. Like the Quartz assemblies, and unlike the Type I assemblies, the Type II assemblies utilize a single blown quartz element that serves as the chimney and has ports for the plasma and auxiliary gas. Like the Type I assemblies, and unlike the Quartz assemblies, the Type II assemblies also have a separate, segmented attachment capable of accommodating alumina injector tubes. One example of a Type II assembly is the Type II Demountable torch, offered as part of a retrofit kit, PE No. 58-0538, manufactured by Perkin-Elmer. An example of a Type II assembly is illustrated in FIGS. 3A-I, 3A-II and 3B.
As illustrated in FIG. 3A-I and 3A-II, the Type II assembly comprises a quartz chimney having ports for plasma and auxiliary gas; an injector support adapter; a mount for the support adapter; and an upper torch clamp. The seal between the quartz chimney and the support adapter is obtained by applying coaxial pressure to the chimney by inwardly compressing the upper torch clamp and forcing the upper torch clamp towards the mount by inserting compression screws into threaded bores. The downward pressure generated by the upper clamp forces the base of the chimney against the upper O-ring, compressing the O-ring an producing a seal.
The injector tube for the Type II assemblies is slightly different than that used in the Quartz of Type I assemblies.; as illustrated in FIG. 3B. This tube is mounted in the Type II assemble by inserting it from the top (instead of the bottom as for the other prior art assemblies). A single O-ring inside the support adapter is responsible for establishing a proper seal.
The Type II assemblies suffer from many of the shortcomings associated with the other prior art assemblies. Initially, as with Type I assemblies, the integrity of the seal between the chimney and the support mount depends on the amount of pressure applied to an O-ring by the quartz chimney. As discussed above, this arrangement often results in improper seals or breakage of the chimney because too little or too much pressure is applied to the quartz chimney. Additionally, Type II assemblies utilize an slitted upper torch clamp that is secured by two screws. Because of the slit in the clamp and the use of only two screws, a non-uniform source of pressure is applied to the chimney, and thus to the sealing O-ring. Unless extreme care is taken in assembling this device, non-uniform pressure is applied to the O-ring resulting in auxiliary gas leaks.
A further problem with the Type II assemblies is the use of a single O-ring, positioned in the upper portion of the support adapter. As discussed above, the use of a single O-ring to seal the injector tube to the support mount necessitates disassembly of the entire torch whenever one O-ring fails. Further, for the Type II assembly, the injector tube O-ring is located proximate to the region where the sample stream exits the injector tube and where the plasma gas enters the torch. Again, exposure to the UV radiation and heat generated in this region tends to degrade the O-ring. Past experience with Type II assemblies indicates leaks generally occur after one to two weeks of use due to heat and UV degradation of the injector tube O-ring.